1. Field of the Invention
This invention relates to an acoustic surface wave device, in which influences due to mechanically reflected acoustic surface waves are reduced.
2. Description of the Prior Art
An acoustic surface wave device is so constructed that electric signals are transformed into acoustic surface waves by a transducer formed on a piezoelectric substrate and that the acoustic surface waves propagate along the surface of the substrate, by using piezoelectric material, such as rock crystal, LiNbO.sub.3 (lithium niobate), etc., piezoelectric ceramic material, or a piezoelectric thin film disposed on a non-piezoelectric substrate. Acoustic surface wave devices are utilized more and more widely as diverse electronic parts, particularly as filters.
FIG. 1 shows an example of an acoustic surface wave device used as a filter, in which 1 denotes a piezoelectric substrate; 2 represents an input transducer consisting of a pair of comb-shaped electrodes 2A and 2B engaging with each other; and 3 represents an output transducer consisting of a pair of comb-shaped electrodes 3A and 3B engaging with each other. The device is so constructed that electric signals coming from a signal source 4 and applied to the input transducer 2 are transformed into acoustic surface waves, which propagate along the surface of the piezoelectric substrate 1, and after having reached the output transducer 3, are transformed again into electric signals and output from a load 5.
The width W of each member electrode of the comb-shaped electrodes and the distance L between two adjacent member electrodes belonging to different comb-shaped electrodes 2A, 2B or 3A, 3B forming the input or output transducer 2, 3 are equal to a predetermined value .lambda..sub.0 /4, where .lambda..sub.0 is the wavelength of the center frequency f.sub.0 of used acoustic surface waves, i.e. the electrodes are formed as normal type electrodes.
A filter provided with transducers consisting of normal type electrodes, as indicated in FIG. 1, has a disadvantage that ripple-like variations in frequency characteristics are inevitable, because there are provoked considerable mechanically reflected waves due to the fact that acoustic surface waves reflected by the extremity of the member electrodes are in phase.
In order to reduce such influences due to reflected waves, a double electrode type transducer shown in FIG. 2 has been proposed. This structure is so designed that each finger-shaped member electrode of the parts of the comb-shaped electrodes 2A, 2B and 3A, 3B is divided into two parts. Owing to this arrangement, the phase of the waves reflected by the extremity of one of the two parts differs by 180.degree. from the phase for the other so that they are compensated by each other and influences due to the reflected waves are diminished.
However, considering that the frequency in recent surface wave devices tends to increase more and more, and hence the wavelength .lambda..sub.0 becomes more and more smaller with increasing frequency, high precision is required for fabrication techniques for the comb-shaped electrodes. For example, using lithium niobate, which is used most widely as peizoelectric substance, and a center frequency of 1 GHz, then .lambda..sub.0 /4=0.87 .mu.m, .lambda..sub.0 /8=0.44 .mu.m. These distances are difficult to realize with a good reproducibility, even by using the newest microfabrication techniques, and it is inevitable that production yield is lowered.
In order to remove these inconveniences, further single phase type transducers, as shown in FIGS. 3(a) and (b), have been proposed. These transducers are constructed using an elastic substrate 6 and a piezoelectric film 7 disposed thereon as a piezoelectric substrate 1, a lower electrode 8 being disposed on said elastic substrate 1 so that the comb-shaped electrodes 9A and 9B, for which the width of each of the member electrodes and the distance between them are equal to .lambda..sub.0 /2, are disposed on the piezoelectric film 7 opposite to the lower electrode 8.
According to this construction, when a signal source 4 is connected between the upper electrode 9A and the lower electrode 8, which is earthed, stress is produced by an electric field due to a voltage applied to the upper electrode. Acoustic surface waves are thus excited and propagate along the direction indicated by the arrow. Since the width of each of the member electrodes belonging to only one comb-shaped electrode and the distance L between them are equal to .lambda..sub.0 /2, requirements on the precision in fabrication techniques are alleviated, and it becomes possible to increase the production yield.
However, this structure is a so-called unbalanced power supply type, in which signal potential is applied to the upper electrode with respect to the lower electrode 8 at earth potential.
Due to this fact, so-called field feed through phenomena are provoked, by which electric input signals are not transformed into elastic surface waves, but transmitted as direct waves. Consequently, in this device, signals due to field feed through phenomena exist together with signals due to acoustic surface waves between the output electrode 9B and the lower electrode 8. The device has, therefore, a disadvantage that its transmission characteristics as a filter are worsened.